Streptococcus mutans, etiologic agent of dental caries, is divided into 5 serotypes. We have isolated and characterized antigens possessing determinants for serotypes alpha and beta. The beta antigen is a wall-localized glycerol teichoic acid and cross-reacts with heart-reactive antibody in human rheumatic sera. Rabbit anti-beta sera contain antibodies that bind to human myocardial sections. The alpha antigen and anti-alpha sera do not display the above cross-reactive properties. These findings dictate caution in the use of anticariogenic vaccines, and suggest that special oral prophylactic procedures may be required for rheumatics. During the coming (renewal) period, we intend to determine the ubiquity of the beta GTA antigen. Many reports in the literature indicate anti-beta sera cross-react strongly with certain other viridans and hemolytic streptococci. We plan to screen cultures for such activity, and isolate and characterize the common antigen by our described methods. Each purified antigen will be assayed for reativity with human autoantibody, and homologous rabbit sera tested for possession of heart-reacctive properties. The discovery of this unique, common antigen in many other streptococci may affect present modes of antibiotic therapy for rheumatics.